This invention relates generally to techniques for using secondary surveillance radar to identify and determine the location of a target such as an aircraft.
Secondary Surveillance Radar (SSR) is a radar system used in air traffic control (ATC). SSR not only detects and measures the position of aircraft, but also requests additional information from the aircraft itself such as its identity and altitude. Primary radar systems measure only the range and bearing of targets by detecting reflected radio signals, which is somewhat like seeing an object in a beam of light. SSR relies on its targets being equipped with a radar transponder that replies to each interrogation signal by transmitting its own response containing encoded data. SSR is based on military Identification Friend or Foe (IFF) technology originally developed during the Second World War, and the two systems are still compatible today. The system has evolved such that the term “identify friend or foe” (IFF) commonly refers to all modes of SSR operation, including civil and foreign aircraft use.
The SSR antenna is used to transmit the interrogation calls and to receive the aircraft data. Military and commercial aircraft have transponders that automatically respond to a signal from the secondary surveillance radar interrogation with an identification code and altitude. The code is a predetermined message in response to a predefined interrogation signal. Before an aircraft begins a flight, it receives a transponder code from an air traffic controller. Normally only one code will be assigned for the entire flight. These codes are sometimes called mode codes. The range to the target is calculated from the time delay between the interrogation and the response time. Thus the SSR system provides for friendly aircraft, all the data that primary radar can provide, and more.
The transponder on an aircraft has an omni-directional antenna so that it can receive and reply to a radar signal from any direction. The transponder receives the signals from the interrogator and selectively replies with a specific pulse group (code) only to those interrogations being received on the mode to which the transponder is set. These replies are independent of primary radar returns, which are received from the target “skin” return. The replies processed by the SSR interrogator for display are sometimes called “plots.” The radarscope used by air traffic control personnel displays returns from both the primary radar system and the secondary radar system. These returns are what the controller refers to in the control and separation of air traffic.
The transponder used in SSR is a radio receiver and transmitter that receives on one frequency (1030 MHz) and transmits on another frequency (1090 MHz). This type of transponder is called a cross-band beacon. The target aircraft's transponder replies to signals from an interrogator (usually, but not necessarily, a ground station co-located with a primary radar) by transmitting a coded reply signal containing the requested information. SSR continuously transmits interrogation pulses as its antenna rotates, or is electronically scanned in space. The reply sent depends on the mode of interrogation. The aircraft is then displayed as a tagged icon on the controller's radar screen at the calculated bearing and range. An aircraft without an operating transponder still may be observed by primary radar, but would be displayed to the controller without the benefit of SSR derived data.
Both the civilian SSR and the military IFF have become much more complex than their wartime ancestors, but remain compatible with each other. One reason for this compatibility is to allow military aircraft to operate in civil airspace. SSR can now provide much more detailed information, and it also permits the exchange of data directly between aircraft for collision avoidance. Given its primary military role of reliably identifying friends, IFF has much more secure (encrypted) messages to prevent ‘spoofing’ by the enemy, and also is used on all kinds of military platforms including air, sea and land vehicles.
There are several transponder modes, each offering different information. Mode 1 provides 2-digit 5-bit mission code (military only—cockpit selectable). Mode 2 provides 4-digit octal unit code (military only—either set on the ground or changed in flight depending on the particular aircraft type). Mode 3/A provides a 4-digit octal identification code for the aircraft, known as a squawk code, assigned by the air traffic controller (military and civilian). Mode 4 provides a 3-pulse reply to a coded challenge (military only). Modes B and D, although originally defined, have never been used for civil ATC purposes.
For civilian flights the modes of operations are A, C and S. Mode S is a relatively new IFF procedure for both military and civilian air traffic control that includes transmission of other data in addition to the mode code. The A mode is based on a 4-digit code using numbers between 0 and 7 assigned by the air traffic controller and set by the pilot to enable identification and monitoring. Mode C transmits pressure altitude that is read automatically from the aircraft altimeter. Mode S is triggered by a mode-S interrogation and can provide the particular information that is requested by the interrogation signal. For modes A and C, all aircraft receiving the interrogation signal will reply, whereas mode S allows aircraft to be addressed individually. In modern ATC systems the data appears with alphanumeric characters in a tag or label linked to the flight position symbol on the radar screen.
Mode 5 provides encryption to provide secure transmission of automatic dependent surveillance broadcast (ADS-B) and GPS position (military only). Mode 5 IFF systems allow for much more sophisticated data exchanges between the interrogator and transponder than other modes allow and have encryption of both the interrogation and the response. With longer interrogation sequences and reply sequences, and improved encryption techniques, the possibility of breaking the encryption by monitoring the communications is greatly reduced.
The level of a transponder is an important feature. The level reports the capabilities of the transponder. A level 1 transponder is one with basic surveillance capabilities. A level 1 transponder will have no provision for datalink capabilities or extended length messaging. A level 2 transponder has all the features of a level 1 transponder with capabilities for Airborne Collision Avoidance System (ACAS), ground station request for altitude and ground station request for the airframe identity. Most transponders installed today are level 2 capable.